The invention relates to a stretcher, i.e., belt tightner for a transmission belt and particularly but not exclusively for a belt for transmitting motion from the drive shaft of an internal combustion engine to auxiliary elements driven by the said engine.
Stretchers are known which comprise a fixed part suitable for being rigidly connected to a support structure integral with the engine block, a movable part hinged to the fixed part about a first axis, a pulley carried by the movable part and rotating with respect thereto about a second axis distinct from the first axis and parallel thereto, and a spring interposed between the fixed part and the movable part, which loads the movable part in the direction tending to keep the pulley in engagement with the relative belt.
Variations in belt tension due to the selective actuation of the various accessories and to wear are therefore compensated by consequent variations in the deformation of the spring and corresponding movements of the axis of rotation of the pulley.
Because the belt transmits a pulsating load to the pulley, to limit the oscillations and prevent the occurrence of resonance phenomena, damping means interposed between the fixed part and the movable part are generally provided.
According to a known embodiment, such damping means comprise a pair of friction discs which are integral with the fixed part and the movable part respectively, face to face with each other and axially loaded by a spring so as to cooperate with each other with respective annular friction surfaces preferably having a conical shape.
The known stretchers of the type briefly described have a number of disadvantages.
In particular, the above-mentioned damping means comprise a large number of components and therefore involve long times and high costs for manufacture and assembly.
Furthermore, the dimensions of the known stretchers are large because it is necessary to provide an adequate support of the movable part with respect to the fixed part to withstand the tilting moments (i.e. about an axis at right angles to the axes of rotation) transmitted to the movable part by the pulley, the latter being supported in a projecting manner.